Complex Urban Systems for Sustainability and Health


Combating high temperatures in cities – the good, the OK, and the ugly

Dr Jonathan Taylor - August 2019

July 2019 was the hottest month ever recorded. While much of the media coverage of heatwaves focuses on the positive – pictures of people in the park, children at the beach, and barbeque summers – there are serious consequences to this heat. For example, during the heatwave in 2003 around 70,000 excess deaths were recorded across Europe due to the high temperatures. While the public health consequences of this past summer aren’t known as yet, it is safe to say it will have a large impact globally. And, with the climate rapidly changing, we need to be prepared for it to get much worse as heatwaves like in 2003 will become the norm. So, how can we adapt our cities to help protect us against this heat? Here are some options, listed from good, ok, and then ugly.

The Good

  • We can learn a great deal from how houses are constructed in warmer climates. For example, Mediterranean- style shutters on the windows can help reduce sunlight entering dwellings, and could reduce summertime heat-mortality by around 43% in the West Midlands of the UK, for example.
  • Increasing the thermal mass of buildings – or the ability of the building materials to store heat by having materials such as concrete, brick, or stone rather than timber – means that houses may be cooler in the daytime and warmer at night.
  • Internal shading like curtains can also help but is slightly less effective than external shutters.
  • It’s also really important to make sure that the houses can be properly ventilated, by being able to open windows so that hot air can escape.

The ok

Some potential changes could be really effective but require a bit more thought.

The Urban Heat Island (UHI) is when an urban area is warmer than the surrounding rural area. It has several causes. Urban materials – such as concrete and asphalt – absorb and retain solar energy better than natural materials. They also store less water; whereas normally energy from the sun might cause water to evaporate, without water this energy heats surfaces instead. Tall buildings can restrict wind-flow, which would otherwise help carry hot air away. And, finally, a lot of heat is produced by buildings, industry, cars, and people in cities. There are ways to potentially reduce the UHI:

  • Replacing these man-made surfaces with natural surfaces. Quality greenspace has the benefit of also making people happier, as well as possibly inspiring them to be more physically active. Planting deciduous trees outside homes might have the dual benefit of reducing the UHI and providing shutter-like shading to houses during the summer, and by reducing the exposure to wind which can cause draughts in housing. Green roofs may also reduce the UHI and indoor temperatures. The downside of reducing the UHI is if the outdoors gets cooler, this could also increase the energy needed for heating in temperate climates during the winter.
  • Installing cool roofs on buildings, which reflect more sunlight and absorb less heat than normal roofs. This has been shown to potentially reduce heat mortality by around 15% in the West Midlands, and would also help reduce the Urban Heat Island, too. However, in the UK, the absorbed heat may actually be helpful during the winter, increasing indoor temperatures and reducing the energy required for heating. Initial estimates show that installing cool roofs in all houses in the West Midlands, UK, might increase winter heating by around 5%, which is not ideal. However, in hotter parts of the world that use air conditioning, this winter heating penalty might be offset by the reduced energy required to cool during the summer.

The potential downsides of the two strategies above can be offset by improving the energy efficiency of housing at the same time. This needs to be done carefully – some renovations such as internal solid wall insulation or increased airtightness can risk increasing indoor temperatures. So, energy efficiency upgrades need to be carried out with consideration for any increased risk of overheating. The good news is that the small increase in overheating risk in energy efficient housing is easily offset by adding shutters and shading to the dwelling.

There are other some potential adaptations that have been proposed that I am not entirely sold on.

  • One suggestion has been to pour water on pavements, like in the traditional Japanese technique of uchimizu, or to spray water mist. However, the UK already experiences water shortages under the current climate, and I am unconvinced this is a viable solution for future climates.
  • In principle, I really like the idea of having public ‘cool’ spaces, or buildings that kept at a cool temperature where residents can gather during periods of hot weather. For example, during the heatwave in 2018, when a supermarket in Finland invited customers to a sleepover, it quickly reached capacity. However, those that are the most vulnerable during hot weather – for example the elderly or disabled – might not be capable of leaving their homes and travelling to such locations.

The Ugly

And, now to the ugly. Air conditioning has been described as selfish, and with good reason – it takes the hot air from inside your house, and pumps it outside, making it everyone else’s problem (and makes the UHI worse). What’s more, it consumes lots of energy, increasing carbon emissions and making climate change worse – a positive feedback that we should really be avoiding if we want to have any chance of keeping temperature increases below 2ᵒC. So, unless zero or low carbon energy is available to run air conditioning, it really should be a last resort – installed after all the zero-energy adaptations have already been tried and indoor conditions are still uncomfortable for too many hours during hot weather, or in situations where the occupants are really vulnerable to heat such as in nursing homes.

I’ve focused here on city-level adaptations, but of course there are plenty of things people can do to help stay cool during hot weather, like staying hydrated, dressing to keep cool, and protecting yourself from the sun. Increased frequency and severity of heatwaves is inevitable in the future, and their significant threats to health will cause a shift in public perception from ‘barbeque summers’ to risk awareness. However, the buildings and cities we design today will be the ones we inhabit in the future. Therefore, there is an urgent need to begin designing and adapting cities – from buildings outwards – to be more resilient to high temperatures, with architects and urban planners playing a critical role.